1. Field of the Invention
This invention generally relates to improvements in furnaces known as biomass gasifier combustors in which biomass fuels are burned in an oxygen deficient atmosphere to produce a flammable gaseous combustion product which is thereafter mixed with air, ignited and fully burned to obtain a clean, stable, high temperature exhaust gas which is free of toxic impurities and which may, therefore, be used to dry grain directly or for other purposes where thermal energy is required.
2. Description of the Prior Art
One such furnace known in the prior art is that disclosed in a U.S. patent application entitled "Biomass Gasifier Combustor" filed by F. A. Payne and I. J. Ross on Jan. 18, 1980 and serially numbered 6/113,339, now U.S. Pat. No. 4,334,484. The subject prior art furnace consists of two separate and distinct chambers, namely, a gasification chamber and a gas combustion chamber, linked together by means of a gas exit pipe.
A suitable biomass fuel such as wood chips, corn cobs, peanut hulls or the like is introduced into the gasification chamber by means of a horizontally disposed auger or screw feeder and deposited on a grate. The fuel is ignited in any suitable manner as by means of a rolled paper wick, propane hand torch or the like in a reducing atmosphere which is substantially deficient in oxygen so that said fuel burns only in a limited combustion region on or near the grate to yield a hot, flammable gaseous combustion product known in the trade as producer gas which is thereafter drawn off of an upper portion of the gasification chamber.
A venturi fan and eductor system is employed to draw a sufficient quantity of ambient air into the base of the gasification chamber to permit burning of the fuel on and near the grate and to draw the resulting producer gas out of the upper end of the gasification chamber, through the gas exit pipe and into one end of the combustion chamber. The hot producer gas entering the combustion chamber is mixed with additional ambient air, ignited and fully and completely burned while flowing across the combustion chamber under the influence of the venturi fan to yield a stable high temperature exhaust gas consisting of CO.sub.2, N.sub.2 and water vapor. The resulting exhaust gas is free of particulate matter, tar and other corrosive and toxic impurities and may, therefore, be drawn out of the combustion chamber and used to dry grain by direct passage therethrough rather than being isolated from the grain by means of an inefficient heat exchanger system. The combustion chamber contains a barrier wall therein which forces the ignited mixture of ambient air and producer gas to follow a circuitous path to assure that all of the flammable products will be completely burned by the time a given quantity of the gas has reached the exit port preparatory to entering the eductor system. In this manner, only non-flammable, clean, stable, high temperature gas will be drawn from the combustion chamber.
One difficulty encountered using the subject prior art furnace is the rapid build-up of tar in and along the exit pipe adjoining the two chambers through which the unburned producer gas must flow. Such a tar build-up occurs because particulate matter contained in the producer gas tends to condense or precipitate out of the gaseous mass as the latter cools during its transition through the exit pipe. As a result, the furnace must be shut down frequently so that the gas exit pipe may be cleaned as failure to clean the pipe can lead to total blockage thereof to the flow of gas.
Another difficulty that has been encountered using the subject furnace is its inability to accurately and automatically control the mixture ratio of producer gas and ambient air in the combustion chamber in order to accurately regulate the exit temperature of the exhaust gas and to assure that the exhaust gas does not contain unburned, toxic substances.
Yet another problem that has been encountered using the subject furnace is the dangerous possibility that a fire occurring in the biomass fuel disposed in the combustion chamber may spread through fuel contained in the auger or screw feeder out of the furnace toward and even into the fuel storage means.
Also, it has been found that when used in grain drying operations, the eductor system of the furnace may emit exhaust gas from the combustion chamber which is far too hot to dry grain directly without cooking, cracking and otherwise seriously damaging the kernels.
By means of the present invention, these and other difficulties previously encountered using prior art biomass gasifier combustors have been substantially eliminated.